The present invention relates to a catalyst. In particular, it relates to a catalyst structure having multiple layers.
All known catalytic membrane reactor configurations fall into one of four topological classifications. The first class we refer to as permaselective wall membrane reactors, which use a semipermeable membrane to transport a product or reactant while confining a bulk or homogeneous catalyst behind the membrane. The second class we refer to as tea-bag reactors which have a catalyst sandwiched between two membranes. The third class we refer to as membrane confined catalytic reactors which have a catalyst in the membrane interior. Reactions are catalyzed and products are formed as reactants flow through the interior of the membrane. The fourth class we refer to as surface catalyzed membrane reactors which have a catalytic layer attached to the surface of the membrane to induce reactions that form products at the exterior surface of the coated membrane.
The present invention deals with a new type of surface catalyzed membrane. The new type of surface catalyzed membrane contains a multilayered catalyst structure on the surface of a permeable membrane which does not have physical micropores. This multilayer catalyst structure on a non-microporous permeable membrane has several functional advantages which have not been realized in previous surface catalyzed membranes. The advantages arise primarily because of the multilayered catalyst structure contained in the membrane. The multilayered catalyst structure spatially separates catalytically important functions such as bond activation, transport and product formation. This type of multilayer surface catalyzed membrane structure has not been disclosed in the past. Surface catalyzed membrane structures which have been studied in the past contain a single layer of catalytic material. Often, the single layer catalyst is coated onto a foreign membrane material which can be either microporous or non-microporous. Other single layer surface catalyzed membrane structures which have been studied are formed entirely from one material which acts as both the membrane and catalyst. An example of a non-microporous single layer catalytic membrane formed entirely from one material would be a palladium foil.
Single layer surface catalyzed membranes formed with non-microporous membranes have been primarily used for hydrogenation and dehydrogenation reactions. Some of the earliest suggestions for reactors employing single layer surface catalyzed membranes come from a group in Russia (E. A. Zelyaeva, V. M. Gryaznov, Izv. Vyssh. Uchebn. Zaved., Khim. Tekhnol., 22(6), 684-7 (1979)) which studied the use of pure metal films (usually Pd foils) in hydrogenation and dehydrogenation reactions. The metal film diffusively transported hydrogen through its crystal lattice either away from (dehydrogenation) or to (hydrogenation) the side where reactions with hydrocarbon molecules occurred. Reaction rates described (V. M. Zhernosek, N. Mikhalenko, E. V. Khrapova, V. M. Gryaznov, Kinet. Katal. 29(4) (1979)) are quire low due to the limited permeability of the thick (20-1,000 micron) films employed to transport hydrogen. These surface catalyzed membranes derived some mechanistic advantages from the spatial separation of the catalytically important functions of bond activation and activated species transport. Hydrogen is activated on one side of the membrane and transported through the membrane. This can dramatically change the availability of hydrogen on the catalyst surface. In a conventional catalytic system where hydrogen, reactant and product all compete for the same surface, the hydrogen availability is determined by the competitive isotherm of the species present. Although these types of single layer catalytic membranes can gain some mechanistic advantages from the spatial separation of catalytically important functions, different and more important mechanistic advantages can be obtained using the type of multilayer surface catalyzed membrane described herein. In particular, it will be shown that multilayered catalytic membranes formed on nonmicroporous supports can obtain a degree of poison tolerance not achieved with the previously described single layer catalytic membranes.